Hi, would like to seek for your advice for the below:

1) Usually on how many weeks can do the 3rd trimester scan?
2) Is it possible book for the scan if without doctor referral letter? 
3) How much for the scan fee? 

FYI, currently I'm 28 weeks and I had done my 1st &amp; 2nd trimester at another centre. I'm planning to back Malaysia end of May and do the 3rd trimester scan before I'm back. However, my doctor advice that is too early that I do the 3rd trimester scan. Is it possible can arrange for me if without doctor referral letter? Thanks.